This invention relates to a power converter apparatus for converting A.C. power into D.C. power.
FIGS. 3-6 are diagrams showing a prior-art voltage converter apparatus disclosed in, for example, the official gazette of Japanese Patent Application Laid-open No. 194697/1984. FIG. 3 is the diagram of the main circuit of an A.C. elevator, FIG. 4 is the block diagram of a control circuit for a converter, FIG. 5 is the block diagram of a current controlling minor loop, and FIG. 6 is the diagram of voltage/current waveforms.
In FIG. 3, numeral 1 designates a three-phase A.C. power source, numeral 2 a converter which is constructed of transistors capable of forced commutation and diodes parallel thereto and which is connected to the A.C. power source 1 so as to convert A.C. power into D.C. power, numeral 3 a smoothing capacitor which is connected to the D.C. side of the converter 2 so as to smooth the D.C. power, numeral 4 a D.C. bus, and numeral 5 an inverter which is constructed similarly to the converter 2 and which is connected to the D.C. bus 4 so as to invert the D.C. power into A.C. power and to perform a variable-voltage and variable-frequency control. Shown at numeral 6 is an induction motor which is connected to the A.C. side of the inverter 5 so as to drive the elevator.
In a case where the motor 6 carries out power running, energy for driving the motor 6 is fed from the A.C. power source 1 through the converter 2 and the D.C. bus 4 to the inverter 5, and this inverter produces the A.C. power of variable voltage and variable frequency, which is supplied to the motor 6. Accordingly, the motor 6 is controlled to a torque and a revolution speed as desired and drives the cage (not shown) of the elevator.
On the other hand, in a case where the motor 6 carries out regenerative running, regenerative energy is fed to the converter 2 through the inverter 5 and the D.C. bus 4 by the function of the flywheel diodes of the inverter 5 and is given back to the A.C. power source 1 by the converter 2. That is, the A.C. power source 1 is regarded as an A.C. machine which rotates at a fixed frequency (here, the commercial frequency of the power source), in opposition to the D.C. voltage source, and the electric power is supplied to the A.C. power source 1 by the converter 2.
In FIG. 4, numeral 9 indicates an A.C. reactor which is inserted in each phase of the A.C. power source 1. Alternating-current detectors 10A-10C detect the currents of lines connecting the A.C. reactors 9 and the converter 2 and deliver alternating-current signals 10a-10c as outputs, respectively. A D.C. voltage detector 11 detects the voltage of the D.C. bus 4, and delivers a D.C. voltage signal 11a as an output. A voltage command value-setting unit 12 issues a D.C. voltage command value 12a, which is set at, for example, a value corresponding to the D.C. voltage of the D.C. bus 4 during the stop (no load) of the motor 6. Numeral 13 indicates a voltage-controlled amplifier, and symbol 13a the output thereof. A three-phase sinusoidal-wave generator 14 generates three-phase sinusoidal-wave reference signals 14a-14c which are synchronous to the A.C. power source 1. Symbols 15A-15C denote multiplier units, which produce respective outputs 15a-15c being three-phase sinusoidal-wave current command values. Current-controlled amplifiers 16A-16C produce outputs 16a-16c, respectively. A saw-tooth wave generator 17 generates an output 17a being an output modulation signal. A comparator 18 produces outputs 18a-18f. A base drive circuit 19 for the transistors of the converter 2 produces outputs 19a-19f which are the switching signals of the transistors.
In FIG. 5, numeral 21 designates an adder. Numeral 22 indicates the transfer function of the current-controlled amplifiers 16A-16C, numeral 23 that of the comparator 18, and numeral 24 that of the converter 2. Shown at numeral 25 is an adder. Numeral 26 indicates the transfer function of the A.C. reactors 9, and numeral 27 that of the alternating-current detectors 10A-10C. Symbol V.sub.2ac denotes a voltage on the A.C. side of the converter 2, and symbol I.sub.2ac a current on the A.C. side thereof.
The prior-art voltage converter apparatus is constructed as described above. The D.C. voltage signal 11a corresponding to the voltage of the D.C. bus 4 is checked with the D.C. voltage command value 12a by the voltage-controlled amplifier 13, and the resulting deviation is delivered as the output 13a. Subsequently, the three-phase sinusoidal-wave reference signals 14a-14c are multiplied by the output 13a in the respective multiplier units 15A-15C. That is, the output 13a serves as a value which determines the amplitudes of the three-phase sinusoidal-wave reference signals 14a-14c. The three-phase sinusoidal-wave current command values 15a-15c and the corresponding alternating-current signals 10a-10c negatively fed back are respectively checked by the current-controlled amplifiers 16A-16C, and the resulting deviations are respectively issued as the outputs 16a-16c. The outputs 16a-16c are compared with the output modulation signal 17a by the comparator 18, whereupon the signal 18a-18f which determine the switching timings of the respective transistors of the converter 2 are output to operate the base drive circuit 19. Then, this base drive circuit applies the switching signals 19a-19f to the bases of the respective transistors so that the D.C. voltage of the smoothing capacitor 3 may equalize to the D.C. voltage command value 12a and that the currents may be controlled into the form of sinusoidal waves. That is, the converter 2 is controlled as a sinusoidal-wave pulse-width-modulation inverter of constant frequency.
FIG. 6 shows the voltage/current waveforms of one phase. In a case where the motor 6 has performed the power running until the D.C. voltage of the smoothing capacitor 3 has become lower than the D.C. voltage command value 12a, the current waveform becomes in phase with the sinusoidal waveform of the power source voltage so as to supply electric power from the A.C. power source 1 to the D.C. bus 4. In contrast, in a case where the motor 6 has performed the regenerative running until the D.C. voltage of the D.C. bus 4 has risen above the D.C. voltage command value 12a, the current waveform comes to have the opposite phase to the phase of the sinusoidal waveform of the power source voltage so as to regenerate electric power from the D.C. bus 4 to the A.C. power source 1.
Even in the sinusoidal-wave pulse-width-modulation control, a ripple component corresponding to a pulse-width-modulation frequency is contained in the current due to the saw-tooth wave voltage. However, the A.C. reactor 9 of comparatively great reactance is inserted in each phase of the A.C. power source 1, and it relieves the ripple so as to obtain a smooth sinusoidal current.
By the way, the voltage-controlled amplifier 13 is usually constructed of an integrator in order to improve the response of the control system.
With the prior-art voltage converter apparatus as stated above, when the voltage-controlled amplifier 13 of the A.C. voltage feedback circuit is constructed of the integrator, the output 13a thereof becomes null at the start of the converter 2 because the integrator needs to be reset at that time. Consequently, the three-phase sinusoidal-wave current command values 15a-15c become null. Since the current on the A.C. side of the converter 2 is null at the start thereof, the alternating-current signals 10a-10c become null. Therefore, the outputs 16a-16c of the current-controlled amplifiers 16A-16C become null, and the transistors are switched so as to render the voltage of the A.C. side of the converter 2 null. As a result, an inrush current expressed by V.sub.ac /(j.omega.L) (V.sub.ac : the voltage of the A.C. power source 1) flows on the A.C. side of the converter 2, to incur such a problem that the smoothing capacitor 3 and the voltage converter elements are destroyed due to the rise of the voltage of the D.C. bus 4.